1. Field of the Invention
In one aspect, this invention is a method and system for detecting sub-wavelength size anomalies in a substrate using optical radiation and an array of resonant photonic open feed antenna elements. In another aspect, this invention is directed to a closed flux resonant antenna that may be used in a system for detecting sub-wavelength size anomalies in a substrate.
2. Background Information
On semiconductor substrates, anomalies, such as process contaminants, post-polishing substrate fragments, or voids need to be accurately located and identified in order to obtain the cleanliness required for the next generation of integrated microcircuits. Because of technological advances, the critical anomaly size is currently less than 80 nm. Because of their minute size, such anomalies are beyond the detectability limit of conventional optical beam scattering approaches, which is determined by the diffraction limit: i.e. approximately one third of a wavelength. In the optical range, this is of the order of 150 to 200 nm since the wavelength of light is on the order of 400 to 600 nm. To extract shape information, the object size should be closer to one wavelength. With standard optical scattering approaches, the smallest objects that can be reliably examined thus are approximately 500 nm, significantly larger then the current critical anomaly size of 80 nm. Because optical instruments and sources in the visible range are very mature and reliable, an optical system that circumvents the diffraction limit would be a much more attractive alternative to perform this function than developing new instruments and sources operating at frequencies above the optical range. This is because mature scatterometry systems which operate at higher frequencies and are capable of scanning productions sized wafers do not yet exist.
Investigators have proposed schemes to defeat the diffraction limit by concentrating the radiated energy into subwavelength areas. An early scheme using a tapered fiber probe succeeded in concentrating the illumination to an area approximately one tenth of a wavelength in diameter. This came, however, at the expense of a tremendous degradation in illumination, as a transmission efficiency of 10−5 was reported. Another scheme to defeat the diffraction limit by interposing an open feed bow-tie resonant antenna element in the path of the illuminating wave is described in U.S. Pat. No. 5,696,372, which is incorporated by reference herein in its entirety. This approach resulted in superior transfer of power from an incident wave to an observation region than previous techniques. The open circuited bow-tie antenna effectively blocks a radiated wave and concentrates its energy in the open circuited gap region of the antenna. The result is an illuminated area of the order of one sixth to one tenth of a wavelength across, with a transmission efficiency ranging from 0.5% to as high as 30%.
The techniques used in the prior art generally suffer from very small transmission efficiencies (i.e., the ratio of incident power to power at the tip) because the concentration of the wave energy is achieved by guiding the wave through closed waveguides that are well below cutoff. That is, the waveguide dimensions are so small compared to the wavelength that most of the incident energy is reflected before reaching the observation aperture at the tip. Further, when used with wavelengths in the optical range rather Man in the microwave range, an inductive load connected to the gap in the bow-tie configuration can change the transmission efficiency from 0.5% to nearly 100%. Thus, the proximity of the object to be detected can dramatically alter the transmission efficiency of the probe by changing the reactance in the near field. A second issue left unaddressed, but which is critical to the application of defect detection on large semiconductor wafers, is that the minuteness of the spot-size implies an enormous scanning time as the illuminated region is physically moved over the area to be examined.
Accordingly, a system and method for rapidly scanning large wafer areas to locate and identify defects or debris addressing the drawbacks of the prior art is needed.